The use of additives such as ferrosilicon and ferromanganese alloys is well known in the metallurgical arts. These materials are often sold in the form of master alloys, that is, alloys of these elements with iron in a relatively high alloy percentage (higher than that desired in a final product) for mixing with more iron or another iron alloy to provide the ultimately desired content of manganese or silicon.
These iron-silicon or -manganese alloys are made by various techniques including the use of blast furnaces and electric arc furnaces. In a conventional process for forming an iron master alloy, a metal oxide such as a manganese oxide or silicon oxide in coarse lump form is charged directly in combination with lump coke or coal into a furnace and heated to reduce the oxide and form molten silicon or manganese. This metal (including silicon which is technically a metalloid) is then mixed with an iron melt to form an iron alloy.
Silicon carbide is generally made by the Acheson process which involves the use of a silicon source such as sand or quartz and a carbon source such as coke breeze in an electric furnace. The heating times are very long and the resulting silicon carbide is relatively expensive, coarse and of mixed quality.
In these processes, the reduction reactions are in part between gas phases and the coarse lump material and in part between liquid metal and liquid slag. Chemical reaction rates are slow, requiring very large furnace size. Heat transfer to the reacting liquids is very difficult, requiring expensive electrical energy, and refractory demands are severe.
Unlike iron, which is fully reducible by carbon from the iron oxide to elemental metal without melting, silicon and manganese are reducible by carbon only at temperatures above their melting points. Thus, while agglomerates of finely divided iron oxide and carbon can be completely reduced in the solid state, similar agglomerates of silicon or manganese oxides will undergo melting and disintegration before reduction is complete and therefor require containment within a refractory vessel and the process in general reverts to a conventional liquid state reduction reaction process with slow reaction rate and expensive heating throughout the process. In addition, such processing normally dictates batch mode which further increases energy costs, material handling costs and decreases productivity.